The Atom

Chapter 8

Great Idea:All of the matter around us is made of atoms, the chemical building blocks of

our world

Chapter Outline

• The Smallest Pieces• The Structure of the Atom• When Matter Meets Light• The Periodic Table of the Elements

The Smallest Pieces

The Smallest Pieces

• Democritus– Cut matter to reach smallest piece– Called “the atom” or “uncuttable”– All material formed from atoms

Dividing a Bar of Gold

Elements

• John Dalton– Father of modern atomic theory– Cannot break down elements– Elements composed of atoms

•Example: Water molecule is 1 oxygen atom and 2 hydrogen atoms

Envisioning Atoms

Are Atoms Real?

• Evidence for the reality of atoms– Behavior of a gas– Chemical combinations– Radioactivity– Brownian motion– X-ray crystallography– Atomic-scale microscopy

The Behavior of a Gas

• Bernoulli – Atoms have mass and velocity and thus kinetic energy

• Decreasing volume increases pressure

• Increasing temperature increases pressure

The Behavior of Gases

Relationship between Volume and Pressure

Volume

0 2 4 6 8 10 12 14

Pre

ssu

re (

mm

Hg

)

10

20

30

40

50

60

70

80

90

Relationship between Temperature and Pressure

Temperature

220 240 260 280 300 320 340 360 380 400 420 440

Pre

ssu

re

30

35

40

45

50

55

Chemical Combinations

• Dalton applied law of definite proportions– Elements combine in a specific ratio of weights•Ex: Water is 8 parts oxygen to 1 part hydrogen

Chemical Combinations (cont.)

• Ratio of weights is a small whole number– Ex: 12 lbs carbon can combine with either 16 lbs or 32 lbs of oxygen

• Implication: some units of elements are fundamentally indivisible

Radioactivity

• Radioactivity– Discovered in 1896

• Phosphors flash when hit by radiation

• 1903 demonstration of the twinkling caused by this effect

Brownian Motion

• Brownian motion: erratic, jiggling motion– Pollen grains suspended in water

• Einstein: motion is random collisions of atoms– Predicted particle movement

• Perrin tested and confirmed Einstein’s predictions

X-ray Crystallography

• Developed in 1912• Demonstrates arrangement of atoms in crystals

Discovering Chemical Elements

• Electrolysis• Mendeleev• Current periodic table

– 112 elements•92 found in nature

• Helium, carbon, aluminum, copper, gold

Everyday Elements

The Structure of the Atom

The Structure of the Atom

• Joseph Thomson identified electron– Negatively charged– Smaller and lighter than smallest atom

• Atoms are NOT fundamental building blocks, but are made up of smaller more fundamental particles

The Atomic Nucleus

• Ernest Rutherford determined atomic structure

• Concluded– Atom has nucleus at center surrounded by electrons

• Later discoveries found nucleus is composed of protons and neutrons

Rutherford’s Experiment

Important Terms Related to Atoms

Why the Rutherford Atom Couldn’t Work

• Why?– Object in circular orbit is accelerating

– Accelerated electrical charge emits electromagnetic radiation

– Electrons giving off energy while orbiting•Result: electrons spin toward nucleus and eventually atom ceases to exist

• Rutherford atom exists <1min

The Nuclear Regulatory Commission

• Logo: highly stylized atomic model

When Matter Meets Light

Niels Bohr

The Bohr Atom

• First working model of atom • Energy levels for electrons

– Specific distances from nucleus– Electrons exist with no radiation– Electrons cannot exist between allowed distances (energy levels)

Bohr Atom

Electron Energy Levels

Atomic Model

Photons: Particles of Light

• Photon absorbed– Used to move electron to a higher energy state

• Photon emitted as electron moves to lower energy state

Photons: Particles of Light

Photons - cont.

• Quantum leap or jump -electron disappears from original location and reappears in final location-never at positions in between

Energy in Bohr Atom

• Energy required to leave ground state– Absorb photon– Heat

•Increases collisions

Spectroscopy

• Atoms emit and absorb different photons

• Depends on differences between energy levels

• Each atom has distinct set of photons

• Spectrum: all photons emitted by an atom– Used for identification

Na, K, and Li – Give Distinctive Colors to a Flame

Spectroscopy – cont.

• Glass prism

Spectroscopy – cont.

• Line spectra for H, Na, and Ne

The Periodic Table of the Elements

The Science of Life

• Spectra of life’s chemical reactions– Used to determine how enzymes work– Used to identify pollutants emitted from automobile exhaust pipes

Science in the Making

• Helium– Discovery– Properties

Technology

• Lasers– Produce a beam of light

Action of a Laser

Lasers Have Many Uses

Periodic Table of the Elements

• Systematizes elements• Columns contain similar elements– Alkali metals: soft, silvery metals

– Noble gases: colorless, odorless, and inert

Periodic Table of the Elements

Why the Periodic Table Works: Electron Shells

• Patterns mirror arrangement of electrons in shells

• Pauli exclusion principle • First shell 2 electrons, second shell 8, etc.

Electron Representation